FAULT TOLERANCE IN

WIRELESS SENSOR NETWORKS

Balaaji Tirouvengadambtirouve@uottawa.ca

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Outline

Introduction FT , WSN

Stages in forming WSN FT Algorithms for different stages Summary Questions

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Introduction

Wireless Sensor Networks Application specific

Fault Tolerance Ensures Reliability

Why FT in WSN Failure is part of the system

Limited power, harsh environment … Malicious

FT parameter – varies with its applications

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Faults in Sensor Networks

Fault Types

Hardware Faults

Software Faults

Application

Middleware

Communication Link Faults

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Fault Detection Self Diagnosis

e.g. Battery depletion Cooperative Diagnosis

e.g. Failure in communication link Fault Recovery

Uses the redundant element (common practice) E.g. Memory, link, secondary power source etc., Deploying redundant sensing nodes Multipath routing – for k path, the system is k-1

fault tolerable12/5/2011

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Stages in forming WSN

Node Placement

Topology Control

Target / Event detection

Data Aggregation

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Node placement

More crucial for immobile nodes

Determines the system

Robustness

Efficiency & performance Architecture

Flat network Two tier

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FT – Node placement(1)

Two tier network architecture Sensor Relay Nodes Sink Relay Nodes (RN)

Should be fault tolerant Form the back bone of the WSN There should be at least 2 disjoint path between

two RN Minimum RN will be chosen to keep the network

k-connected Each sensor connects to at least one RN Sensor does only sensing, routing / data

forwarding is done by RN Increases Sensors operational life12/5/2011 8

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Topology Control

Node Placement is not always the final

solution

Topology Control is must to maintain the

fault tolerance of the system

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FT – Topology Control (1)

An algorithm is proposed by Yong Chen et al. in [3] A CDS graph is constructed for a system CDS forms the backbone network For each node in CDS, it adds the neighbour of the

node to the backbone to meet required vertex connectivity

Power on/off model is applied to the so formed model Redundant nodes in the backbone goes to sleep

mode, thus maintaining k-vertex connection Fair rotation among active and sleep nodes So any point of time k-vertex connection is maintain

thereby allows the system to work even if (k-1) node fails

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FT – Topology control (2) A FT TC algorithm was proposed for

heterogeneous network by M. Cardei et al. in [4]

Heterogeneous network ? WSN with mixed hardware capabilities Some nodes are more powerful than other

nodes Super Nodes (SN) More communication range, higher power …

Their model Assumes SN SN is highly reliable

They address only sensor to sensor & sensor to SN12/5/2011 11

Heterogeneous Network (WSN)

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[4]

M. Cardei et al. constructed a reduced graph with directed edges for the system

The proposed 3 algorithm which can be applied on top of the derived reduced graph

For simplicity, only one algorithm is discussed here.

Minimum Weight-Based Anycast Topology Control [4] There are k-vertex disjoint path between a

given vertex and any other vertex in the subgraph

the sum of the weight of the selected edges is minimized.12/5/2011 13

FT TC (2) – Reduced Graph

In Figure square : SN circle : sensor

SN can communicate each other, so can be merged to one node : root node

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[4]

Rules for adding directions to the edges Edge between sensor

replaced with bidirectional edge with same weightage

Edge between sensor and root node Replaced with unidirectional edge towards root

node Only the edge to the closest SN is kept

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[4]

FT – Topology Control (3)

Shantanu Das et al. proposed a FT topology control approach in [5] This can be employed in networks which has

limited mobility to the sensor nodes The intention of the author

To remove all critical nodes Single point of failure

To make the network bi-connected More fault tolerant Sub graphs are interconnected by two nodes

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P-hop sub graph In the given sub graph any node can reach

any other node within p hops P-hop critical node

A node which make the P-hop sub graph disconnect on its failure

Bi-connected network Not even one p-hop critical node All critical nodes non critical nodes

Asks two of its neighbour node to move closer to each other to establish a redundant link

This movement can break an existing link

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Three possibilities can exists in a network Critical node with

No critical neighbour ( all its neighbour are non critical nodes)

One critical neighbour More than one critical neighbour

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Critical node with no critical neighbour – 1/2

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Critical node with no critical neighbour – 2/2

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Critical node with no critical neighbour:

The critical node identifies the two disconnected sub graph

Chooses one node from each graph such a way that those two nodes can move closer to establish a new link

The neighbour node are chosen such a way that the existing links are not much disturbed (some case disconnection of some non critical node cannot be avoided)

Since the two nodes make another path, the critical node will now become non critical

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Example showing node movement causing disconnection

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Critical node with one critical neighbour – 1/3

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Critical node with one critical neighbour – 2/3

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Critical node with one critical neighbour – 3/3

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Critical node with one critical neighbour:

Critical node with larger node id is chosen first It will try to relocate its non critical node to

establish a link Now the critical node with higher node id will

become non critical Repeat above process till critical node with no

critical neighbour case is reached.

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Critical node with several critical neighbour:

Critical node Available – if it has a non critical neighbour Non available – if it does no have any non critical

node Critical head

Critical node with higher node value among available critical node

Pairwise merging strategy Critical head dominates pair merging and selects

one of its critical neighbour to pair with it.

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Critical nodes1 (3 is unavailable), 5, 6

Pair : (1.3) (5,4) (6,4)Dominated node : 1, 5, 6

The steps are repeated

FT – Target / Event Detection

The assumption is that in a dense sensor network The event region will be shared by many sensors The data among the sensors are collected and

compared to derive a decision. The faulty sensor reading will be masked by the

majority of non faulty sensors There are many techniques used in

literatures Calculating the median for a group of sensors Calculating the failure probability of a node to

decide on its future decision correctness

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FT – Data Aggregation

Reliable aggregation in Track Topology

Edges : Primary, backup & side edge The back up node just listens Each message is attached with a bit vector, informing

about faulty links Backup node aggregates the received data along with

its on seeing a error bit from the side edge 12/5/2011 29

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Summary

Fault Tolerance is very crucial for sensor networks

Many algorithms have been proposed in literature addressing the FT issues

Few of FT algorithms were discussed

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Question #1

Construct a reduced graph and show the directed edges in it using the following figure

Note: The black square represents the super nodes, which can communicate each other with high reliability (a message sent to any super node is considered that all other SN will receive it)

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Answer #1 Reduced Graph

Directed reduced graph

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Question #2 The below shown figure is 2-hop network with 4

and 5 as its critical nodes, Assume the nodes can be moved, use node mobility to remove node criticality and derive a bi-connected network (Note: critical node id with higher value will dominate the lower value, assume the node mobility is not breaking any of the existing connection)

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Answer #2

Moved nodes : 7 and 1

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Question #3 Describe the steps involved in converting

critical node into non critical node, when a critical node has several critical nodes as its neighbour

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Answer #3

Identify the Available and non available critical nodes

Critical nodes with higher node id than its neighbouring available critical node will declare itself as Critical head

Critical head pairs up with its neighbour critical node and dominates the pair to remove criticality

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References

[1] “Guide to Wireless Sensor Networks”, Chapter 10 Fault-Tolerant Algorithms/Protocols in Wireless Sensor Networks, Hai Liu, Amiya Nayak, and Ivan Stojmenovi´c

[2] J.L. Brediny, E.D. Demainez, M.T. Hajiaghayiz, and D. Rus, “Deploying Sensor Networks with Guaranteed Capacity and Fault Tolerance,” MobiHoc 2005, urbana-champaign, IL, 2005

[3] Y. Chen, and S.H. Son, “A Fault Tolerant Topology Control in Wireless Sensor Networks,” Proceedings of the ACS/IEEE 2005 International Conference on Computer Systems and Applications, 2005

[4] M. Cardei, S. Yang, and J. Wu, “Algorithms for Fault-Tolerant Topology in Heterogeneous Wireless Sensor Networks,” IEEE Transactions on Parallel and Distributed Systems, vol. 19,no. 4, pp. 545–558, 2008

[5] S. Das, H. Liu, A. Kamath, A. Nayak, and I. Stojmenovic, “Localized Movement Control for Fault Tolerance of Mobile Robot Networks,” The First IFIP International Conference on Wireless Sensor and Actor Networks (WSAN 2007), Albacete, Spain, 24–26 Sept. 2007

[6] S. Gobriel, S. Khattab, D. Mosse, J. Brustoloni, and R. Melhem, “Fault Tolerant Aggregation in Sensor Networks Using Corrective Actions,” Third Annual IEEE Communications Society on Sensor and Ad Hoc Communications and Networks, vol. 2, pp. 595–604, 2006

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Thank you !!!

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